The invention concerns in particular a process for the extrusion blow moulding of thermoplastic material to form integral hollow bodies, in particular fuel tanks of plastic material for motor vehicles.
It is basically known for extrusion blow moulding of hollow bodies to be effected by means of extrudates in web or band form. They can be obtained for example by the extrusion of a tubular preform which was cut up or divided into bands or webs. The extrusion of extrudates in web and band form without implementing the roundabout route of using the tubular form is also known. The preforms are either removed at the extruder by means of a manipulator and fed to the tool or they are extruded directly between the opened portions of a tool.
In the production of fuel tanks or other relatively large containers which are provided with built-in fitment components and attachments, the procedure involved is normally such that a tubular preform is extruded in a hanging condition, in the form of a multi-layer extrudate, either continuously or discontinuously, wherein after being expelled to the predetermined length the extrudate is shaped directly within a tool formed from two blow moulding mould halves. Expansion of the preform is effected by the preform being subjected to the action of gas pressure within the closed blow moulding mould.
If built-in fitment components such as for example surge components, fuel pumps, senders, roll pots and the like are to be fitted in the fuel tank, either they can be fitted by the plastic material being blow moulded therearound in production of the hollow body or they can be subsequently fitted into the finished container through assembly openings on the container. The latter procedure is less desirable for various reasons. More specifically, for emission reasons, openings and cut-out orifices in the container wall should be as far as possible avoided or kept as small as possible. In addition the manual assembly expenditure on a fuel tank produced in that way is comparatively high.
The operation of introducing such built-in fitment components is found to be particularly difficult when the fuel tank is comparatively rugged and irregular, that is to say it is of a complex spatial configuration.
Therefore, as an alternative to integral containers which are produced by extrusion blow moulding, the possibility is known for producing such containers, for example fuel tanks, from two injection-moulded or deep-drawn half-shell portions. That procedure affords the advantage that built-in fitment components can be fitted into the opened half-shell portions. The two half-shell portions are then welded together to produce the closed container. In that case the operation of assembling built-in fitment components can be very substantially automated. A problem with fuel tanks which are produced in that way is the weld seams at the connecting locations of the half-shell portions. They represent potential leakage locations on the container for gaseous hydrocarbons and require additional devices for making the welded connections.
In principle there would also be the possibility of placing built-in fitment components in the interior of a container when producing containers by extrusion blow moulding of two or more preforms in web or band form, by means of suitable manipulators. It will be noted however that this is relatively complicated and expensive if the situation involves introducing into the container fitment components which extend over the overall cross-section of the container or which indeed are intended to join the mutually oppositely disposed inside walls of the container together. In that respect the procedure that presents itself for that purpose is to assemble the containers from half-shell portions which are prefabricated, for example injection-moulded or deep-drawn.
The object of the invention is therefore that of providing a novel process for the production of hollow bodies of thermoplastic material by extrusion blow moulding of preforms in web or band form, in which built-in fitment components can be placed in the container, at a particularly low level of assembly complication and expenditure. In particular the invention seeks to provide that the process minimizes manual and machine finishing working on a container provided with built-in fitment components.